The specific aims of this study are to 1) evaluate GFR in cystic fibrosis patients at baseline and after 6 months of ibuprofen (IBU) therapy; 2) to evaluate tobramycin pharmacokinetics in CF patients at baseline and after six months of IBU therapy; and 3) to compare baseline GFR in CF patients vs. historic age-matched controls. The hypotheses are: 1) high dose IBU therapy causes a significant decrease in GFR in CF patients after six months of therapy; 2) clearance of tobramycin is significantly decreased in CF patients after initiation of high dose IBU therapy when compared to baseline values; 3) Baseline GFR in patients with CF is not different from healthy age-matched control patients. This is a recently approved study which is just beginning patient recruitment.